Recently, film bulk acoustic wave devices (FBARs), and band-pass filters and duplexer filters using the same are currently being widely studied and developed as next generation components conforming to the trends of high frequency, high quality and miniaturization of mobile communications components.
Film bulk acoustic wave devices use a piezoelectric/inverse piezoelectric phenomenon of piezoelectric materials. The most ideal form thereof (i.e., air-gap form) has a simple structure of a piezoelectric thin film and an upper/lower electrode. Several methods for manufacturing a film bulk acoustic wave device having an ideal form (i.e., air-gap form), which is simple and has excellent resonance characteristics, have been suggested. The methods may be largely classified into micro electro mechanical system (MEMS) processes, bulk micromachining manufacture methods using backside etching, and surface micromachining manufacture methods based on two schemes of using a sacrificial layer after a substrate groove formation and of using a sacrificial layer on a substrate. In addition, there are device manufacture methods, such as solidly mounted resonator (SMR) methods using a Bragg reflector, high-overtone BAR (HBAR) methods using a low-loss substrate, and the like. Primarily, the surface micromachining method using a groove is being used. The SMR device is currently studied by some manufacturers. For the formation and removal of the sacrificial layer in the surface micromachining method, there are several patents on detailed techniques. For example, there are a number of methods, such as a method for forming and wet-etching a resonance structure outside a ZnO sacrificial layer; a method for laminating a sacrificial layer, forming a step-shaped support layer thereon, forming a resonance structure, and then removing the sacrificial layer; a method of manufacturing a resonator using a bridge structure, a method for forming a groove and forming a sacrificial layer therein, a method using deep silicon etching equipment, and the like.
A conventional method of manufacturing a film bulk acoustic wave device using the aforementioned surface micromachining process is a method of forming a groove having a depth of a few micrometers (μm) (i.e., an empty space functioning as an acoustic reflective layer) on a silicon substrate, depositing a sacrificial layer such as SiO2 (BPSG, low temperature oxide (LTO)), Poly-Si, ZnO, etc. in a thickness of a few μm in the groove, mirror surface polishing the layer using a chemical mechanical polishing (CMP) process, and forming a film bulk acoustic wave device having a support layer/lower electrode/piezoelectric thin film/upper electrode structure thereon.
In the foregoing, in order to mirror surface polish the sacrificial layer, there is necessarily a need for a CMP process of two or more steps. In the first CMP process, planarization is conducted to reduce the step between the sacrificial layer and the etch protective region, and in the second CMP process, the sacrificial layer is mirror surface polished. This is because the surface of the sacrificial layer beneath a piezoelectric layer must be extremely smooth in order to reveal excellent piezoelectric characteristics by improving the c-axis preferred orientation of the piezoelectric layer, which is an essential part of the film bulk acoustic wave device formed by the CMP process.
Here, chemical mechanical polishing (CMP) refers to a process of planarizing a substrate or a sacrificial layer using both chemical reaction and mechanical polishing. A polisher is attached to a flat platen of high precision, slurry with polishing agent and chemical polishing solution being mixed is poured thereon, and a machined object is brought into friction with the polisher rotating through the platen rotation. With polishing agents and polishing solution therebetween, the machined object is polished by mechanical friction and is etched by chemical reaction. This mechanical-chemical actions cause a synergistic effect so that the object can be polished to a mirror surface.
A conventional CMP process refers to a process of first forming a groove by etching a substrate, depositing a sacrificial layer in the substrate, and smoothly polishing the entire substrate surface using a CMP process when manufacturing a bulk acoustic wave device. The first CMP process is a rough polishing process. It is a planarization task for causing the surface height of the sacrificial layer deposited in the groove and the surface height of the substrate to be generally similar to each other by removing the sacrificial layer deposited on the substrate other than the groove. The second CMP process is to smoothly polish the sacrificial layer portion deposited in the substrate groove. The second CMP is a very important process. Using this process, the surface of the sacrificial layer in the groove must be made extremely smooth. This is because when depositing any material, the substrate surface state as a base affects all the characteristics of a thin film as deposited. In particular, the c-axis preferred orientation of the piezoelectric element is a very important characteristic of the piezoelectric thin film. This characteristic is under direct influence from the surface roughness of the sacrificial layer. The smoother the sacrificial layer surface, the better the preferred orientation of the piezoelectric element. Accordingly, the device characteristics can be of high quality.
In a conventional method of manufacturing a film bulk acoustic wave device using the aforementioned surface micromachining process, a groove is formed in a portion of a substrate surface, a sacrificial layer is deposited therein, the sacrificial layer other than the groove is removed by a two-step CMP process, and the sacrificial layer filled in the groove remains. The sacrificial layer is a portion to be removed in a final process for the device, which is called ‘sacrificial layer’.
Generally, in a case where a thin film is deposited on the substrate, the thin film may be monocrystalline (epitaxy), polycrystalline, and amorphous. In the case of a sputter deposition, it is easy to obtain a thin film of which the atoms are aligned in one-dimension when in a broad view, which is called epitaxial growth or preferred orientation. Generally, if the forward direction of crystalline directions from an arbitrary origin is set as an a-axis, the rightward direction is set as a b-axis, and the upward direction is set as a c-axis, the c-axis preferred orientation indicates that, when an AlN and ZnO thin film is formed on the substrate, the c-axes of the two materials are oriented perpendicular to the substrate surface.
The c-axis of a piezoelectric material used in the present invention corresponds to a piezoelectric axis (on which electrical and mechanical signals react most strongly). In the film bulk acoustic wave device, the c-axis preferred orientation of the piezoelectric material must be excellent to obtain a good piezoelectric effect of the piezoelectric element and thus to make a device having excellent resonance characteristics using the piezoelectric effect. Thus, the c-axis preferred orientation of the piezoelectric thin film is a very important characteristic.
Herein, a prior art for manufacturing a film bulk acoustic wave device will be specifically discussed. As described above, the manufacture method using surface micromachining is being widely used. This method may be classified into a method using a sacrificial layer after forming a substrate groove and a method using a sacrificial layer on a substrate.
First, the surface micromachining method (surface M/M 1) using the sacrificial layer after forming the substrate groove will be discussed. As shown in FIG. 10, it is a method that uses the sacrificial layer after forming a groove having a depth of a few μm in the substrate surface. This method is of a structure having a simpler process and higher yield as compared to bulk micromachining, and has been considered a substitution technology for the bulk micromachining method because of its mass-production. However, there is a problem with this method that a two-step CMP process is needed, a dishing effect exhibits upon the CMP process, and stress is focused on bent portions, resulting in micro-cracks.
In the case of using the aforementioned surface M/M 1, the CMP must proceed in two steps for smoothness of the sacrificial layer and the substrate surface after a substrate groove is formed and then a sacrificial layer is deposited therein in order to manufacture the film bulk acoustic wave device. In the first step, it is necessary to increase the rate of the CMP for removal of the sacrificial layer on the substrate and for smoothness between the substrate surface and the sacrificial layer surface in the groove. Thus, rough polishing will be conducted. However, if the surface of the remaining sacrificial layer is rough, the c-axis preferred orientation of the piezoelectric thin film is deteriorated when the piezoelectric thin film is deposited on the rough sacrificial layer surface. Thus, since it is necessary to make the surface of the remaining sacrificial layer smooth for the sake of the c-axis preferred orientation of the subsequent piezoelectric thin film, the surface of the remaining sacrificial layer is processed to be further smoothed by adding a two-step CMP process.
However, after the two-step CMP process is undertaken to make the surface of the sacrificial layer smooth, a dishing effect is shown. Here, the dishing effect is generated between materials having different material quality, such as between the substrate and the sacrificial layer. Since the sacrificial layer is a material with weaker property than that of the substrate, it is much more highly polished compared to the substrate at the same polishing condition. The sacrificial layer is much more polished into a sunken shape when viewed from the substrate surface. A heavily sunken layer leads to micro-crack generation, sacrificial layer removal problems, and stiction problems as described below.
Further, all mechanical destruction occurs as cracks propagated from micro-cracks. Accordingly, even though it is important to prevent the propagation of cracks, it is essentially important to block micro-cracks from occurring. Micro-cracks occur most frequently at portions on which the stress of the thin film may be focused. In particular, physically bent portions or interface portions require careful attention because most of the stress is focused thereon. Accordingly, the use of curved portions, rather than bent portions whenever possible prevents the concentration of stress and the occurrence and propagation of micro-cracks. In FIG. 10, there is shown a portion where the substrate remains and a portion where the sacrificial layer is removed into an air layer. Therefore, in the process of removing the sacrificial layer, stress is necessarily generated at its interface and such a portion will physically be weak.
Next, the surface micromachining method (surface M/M 2) using the sacrificial layer on the substrate will be discussed. As shown in FIG. 11, it is a method depositing and using the sacrificial layer in a thickness of a few micrometers (μm) on the substrate surface. This method is the same as the surface micromachining method using the sacrificial layer after forming the substrate groove in that the process proceeds near the substrate surface, but is different from it in that the device is manufactured by using the sacrificial layer patterned on the substrate while using the substrate as it is. Such formation of the sacrificial layer pattern on the substrate eliminates the need for a groove to be formed on the substrate, resulting in a shortened manufacture process. However, there is a problem in that a lower electrode is positioned at a sacrificial layer position from the substrate and an upper electrode is positioned at piezoelectric element height from the lower electrode and, as such, the higher the electrode is formed from the substrate surface, the more difficult the process becomes, and a parasitic effect arises. In other words, there is a risk that a portion with a bent electrode has a parasitic effect in a circuitry aspect and causes an electrical short in a physical aspect. Upon depositing a metal, because a top surface and a side surface are not deposited to have the same thickness but the top surface has a much greater deposit than the side surface, there is a likelihood that the side is not connected electrically, resulting in a short-circuit. The surface M/M 2 method has a problem that stress is focused on bent portions, resulting in micro-cracks. In particular, since the bent portions are present at two places of the lower electrode and the upper electrode, an inferior physical structure is obtained.
Further, in the case of using the surface M/M 2, a stiction risk arises upon removing the sacrificial layer. Stiction means that the thin film remaining after the sacrificial layer removal and the substrate portion stick to each other. Easily speaking, for example, in water, even though two large-area glass plates are made to face each other, they remain in a separated state. On the contrary, out of the water, the two glass plates strongly stick due to surface tension and capillary action, which may be referred to as stiction. In conventional surface micromachining methods, because the sacrificial layer is present beneath the piezoelectric thin film and on the substrate, this structure is immersed in etching solution, which melts only the sacrificial layer, such that only the sacrificial layer is removed from the structure. If the sacrificial layer is removed and then the device is taken out of the etching solution, the etching solution is gradually removed between the piezoelectric thin film and the substrate and then the piezoelectric thin film and the substrate are strongly stuck to each other. This phenomenon is called stiction. Occurrence of stiction obstructs the normal operation of the device. A process of removing the sacrificial layer without stiction may be the most difficult process in the manufacture of a film bulk acoustic wave device. In the surface micromachining method using the sacrificial layer on the substrate, the sacrificial layer has a height of approximately a few micrometers (μm), the piezoelectric thin film has a thickness of about 1 μm, and the piezoelectric element has a suspending length of about 200 μm. Therefore, there is a problem that when a structure is formed in which the piezoelectric thin film having a length of 200 μm and a thickness of 1 μm is suspended at a position of about 3 μm from the substrate, stiction easily may occur and the piezoelectric thin film may be bent and stuck to the substrate.
In the case of manufacturing a film bulk acoustic wave device using the surface micromachining method as described above, there is a problem that the two-step CMP process is necessary, a dishing effect is shown upon the CMP process, and micro-cracks occur at bent portions on which stress is focused. Further, a parasitic effect and stiction risk upon the sacrificial layer removal may arise. As such, the process is very difficult in the conventional surface micromachining method because the manufacture process is complex, and productivity of the film bulk acoustic wave device is poor because process time is long.
In the case of manufacturing the lower electrode or upper electrode using a single molybdenum electrode as in a prior art, an additional etching process is needed to etch an oxide film of the molybdenum electrode. Further, if the electrode is exposed to the air in a post-process, the electrode is easily oxidized. Accordingly, electrode resistance increases, complete oxidation of the molybdenum oxide is difficult, and re-oxidation may occur. There is another problem that if the molybdenum electrode is over-etched to completely etch the oxide, the mass loading effect causes a heavy shift of the resonance frequency. The use of noble metal material having less acoustic loss, such as platinum (Pt) or the like, as an electrode material to avoid this problem greatly increases production costs.
Here, the resonance frequency of the film acoustic wave device basically depends on spacing between two electrodes, namely, the thickness of the piezoelectric element, and if the electrodes are brought into contact with each other for applying an electrical signal, the electrodes reduce the resonance frequency, which is called mass loading. In general, the resonance frequency lowers and, for a gold (Au) electrode, the resonance frequency sometimes lowers to 60% of the theoretic value. Since an aluminum electrode has a much smaller mass loading effect, the use of aluminum electrodes to obtain the same resonance frequency may result in a thicker piezoelectric element thin film as compared to the use of other material electrodes.
In a conventional process for manufacturing film bulk acoustic wave devices, the sacrificial layer is removed to complete a structure of the film bulk acoustic wave devices, and the film bulk acoustic wave devices formed on the substrate are then cut by a sawing method, wherein a lid-shaped protecting structure must be formed of, for example, a glass plate to protect the film bulk acoustic wave devices. In the foregoing, the protecting structure is needed for preventing cooling water upon sawing or residues created upon cutting from affecting the structure of the film bulk acoustic wave devices. However, there is a need for a process of removing the protecting structure from the respective bulk elastic plate devices after cutting, and this process is very complex, resulting in degraded mass productivity of the film bulk acoustic wave devices.
As another cutting method, there is a bulk micromachining process of forming cracks on the back surface of the substrate prior to a film bulk acoustic wave device manufacture process, and cutting the substrate using the cracks through application of mechanical force after the film bulk acoustic wave device is formed on the substrate. However, even in this cutting method, the process is difficult, the film bulk acoustic wave device is applied with physical impact, and mass productivity is not good.